Imaging the permeability structure within sediments has been a challenge to exploration geophysicists for many years. The results so far have been very limited. In practice, the most reliable method of measuring the permeability within sedimental earth is through the use of a pumping test. This method requires at least two wells, one to constantly pump water out at a constant rate and another to observe the change in the water level. These two wells must also penetrate down to the layer of interest, and the well section above the layer of interest must be cased to isolate the layer. In such conventional systems, the two wells have to be separated by a horizontal distance of at least 500 ft.
However, this method has the significant disadvantage that it is very expensive and time-consuming, and only produces the measurement of an average permeability of the layer between the wells. This method also does not provide any information concerning the spatial distribution of permeability, known as the permeability image.
Cross-well seismic tomography has been widely used to image geological structures within the earth. Usually, the seismic velocity and attenuation information are inverted from measured arrival times and amplitudes of the seismic pulses received in a well, which were originated from another well separated by a certain horizontal distance. Two systems of conducting cross-well tomography are disclosed in U.S Pat. Nos. 5,142,500 and 5,406,530, the contents of which are hereby incorporated by reference herein.
U.S. Pat. No. 5,142,500 discloses a method of measuring the permeability, porosity and shear strength of a geological structure. This patent also discloses how the average permeability of beach sand between a source and a receiver can be measured acoustically by measuring the sound velocity and attenuation at multiple frequencies and comparing the data to the theoretical values of velocity and attenuation at the same frequencies, as calculated using the Biot theory, which is discussed in detail in Biot, M. A., "The theory of propagation of elastic waves in a fluid-saturated porous solid, II high frequency range," J. Acoust. Soc. Am., vol. 28, 179-191, 1956, the contents of which are incorporated by reference herein.
U.S. Pat. No. 5,406,530 discloses a non-destructive system of measuring the range, the accuracy, and the frequency resolution of acoustic cross-well tomography. It dramatically improves the measurement of these characteristics through the use of a pseudo-random binary sequence ("PRBS") method. The invention of PRBS has enabled users of the system to obtain accurate and long distance images of sound velocity and attenuation within sediments. However, while the PRBS method disclosed in U.S. Pat. No. 5,406,530 provides significant advantages in the accuracy of long distance imaging of sound velocity, it produces only the average permeability between a source and receiver by repeating acoustic transmission at multiple PRBS frequencies No spatial distribution or imaging of the permeability structure within the sediments was obtained from that invention, and sound velocity and attenuation still had to be measured at multiple frequencies.
The methods disclosed in the aforementioned patents provide significant advantages over the more conventional methods, such as pumping tests, which are very expensive and time-consuming, but only measure an average permeability of the layer between the wells. The methods disclosed in the aforementioned patents can be even further improved through the use of a cross-well tomography system which uses a single PRBS signal to create a spatial distribution at permeability throughout a geological structure.